Geomorphic controls on debris flow activity in the paraglacial zone of the Southeast Tibetan Plateau

On the southeastern Tibetan Plateau, an area widely covered by alpine glaciers, two types of debris flow generally occur: glacier-related debris flows (GDFs) and rainfall-related debris flows (RDFs). It is widely accepted that topographic conditions influence debris flow activities; however, few studies have examined the differences between such influence on GDFs and RDFs. This study investigated the GDFs and RDFs in the periglacial area of the Parlung Tsangpo Basin, and calculated 12 geomorphic indexes to reveal the topographic features associated with these two types of debris flow. It was found that lower values in the drainage area, main channel length, and relative elevation occurred in RDFs compared to the GDFs. In contrast, higher values in the channel gradient, relief ratio, and effective basin area appeared in RDFs. The discrepancy is mainly related to the different topographic and geomorphic shaping of modern glaciers. According to its geomorphological characteristics, the Parlung Tsangpo Basin can be divided into the upper V-shaped canyon section, middle wide valley section, and lower steep canyon section. The scale and frequency of debris flow in the upstream canyon region are substantially lower than those in the downstream. Moreover, the frequency and scale of RDFs are considerably different from those of GDFs, primarily because of the different geomorphic evolutionary stages of debris flow gullies in different regions.